While the invention is subject to a wide range of applications, it is especially suited for use in a vehicle power steering system and will be particularly described in that connection.
In hydraulic power steering systems of vehicles, the general practice is to have a primary steering pump driven by the vehicle engine and supplying hydraulic fluid to a steering valve which is controlled by a steering wheel. In the event of a failure of the primary pump, fluid is not supplied to the steering valve and manual steering of the vehicle is extremely difficult. Assuming the failure occurs to a large, off-road, mobile construction machine, the operator may be in a hazardous position because of the extreme difficulty in steering due to the loss of power. Consequently, it is a well-known expedient to include a backup system capable of supplying sufficient power to a steering valve in the event of a primary power failure and thereby enable an operator to safely steer the crippled vehicle.
In the past, one attempt to solve the above-mentioned problem was to provide a back-up system which stored emergency fluid in an accumulator. In this type of system, a pressurized accumulator, for example, a cylinder of oil, delivers fluid to a steering valve in the event that the primary pump fails to supply the necessary fluid. Accumulator systems are expensive due to the complex apparatus required. Further they are unreliable as they sit idle whenever the primary pump is working properly, and when the accumulator is finally needed, an unseen defect in the system can easily have occurred. Additionally, accumulators are not practical for providing steering during long distance moving of vehicles which have lost their primary power.
Another solution to the problem of steering failure is the addition of a backup system including a ground driven pump. This pump, driven directly or indirectly by the vehicle wheels, is automatically connected to the steering valve upon failure of the primary pump or the hydraulic circuitry connected thereto. An example of a ground drive pump system is disclosed in U.S. Pat. No. 3,747,725 to Van Wicklin et al. This patent discloses, for example, "in accordance with the present invention, a vehicle power steering system having a primary and a secondary or an auxiliary pump includes a transfer valve for altering the fluid flow circuits of the primary and auxiliary pumps . . . Should the mass flow rate become insufficient in magnitude, the transfer valve disconnects the primary hydraulic supply circuit from the steering assist mechanism and connects the auxiliary hydraulic supply circuit to the steering assist mechanism so that power assisted steering is retained." The auxiliary pump disclosed in this patent is always delivering its total output to the transfer valve whenever a vehicle, which includes this system, is moving.
The problem with a system of the type disclosed above is that a significant hydraulic horsepower loss is created by the flow of hydraulic fluid from an auxiliary pump through a transfer valve. The following equation for hydraulic horsepower loss may be derived from the SAE Handbook, 1973, Procedure J 745 C: EQU hhl = p .times. q/1714
where:
Hhl = hydraulic Horsepower Loss PA1 P = differential Pressure (psi) PA1 Q = flow Rate (gpm)
Thus, a hydraulic horsepower loss is present whenever a fluid from an auxiliary pump crosses a differential pressure in the flow path, such as, for example, the differential pressure across a transfer valve. In a system of the type where the auxiliary pump is always delivering its total output to a transfer valve, the horsepower loss may be significant. With the increased cost in fuel, efficiency of a machine becomes very important.
Another disadvantage of many prior art ground drive pump systems is the complexity of the apparatus. The transfer valve is a separate unit which may receive an input flow from both a main pump and an auxiliary pump and directs the output flow to the proper location. Thus, a transfer valve is relatively large and requires some extensive plumbing for installation. Also, a reservoir in this type of system requires a capacity to handle the total supply capabilities of both pumps.
Another example of a ground drive pump steering system is disclosed in U.S. Pat. No. 3,631,937 to Joyce. This patent discloses, for example, "a supplementary steering system for use with the primary hydraulic steering system of a vehicle having a steering valve. The supplementary steering system includes a supplementary pump driven by the driven shaft of the vehicle and a supplementary valve that discharges the output of the supplementary pump in the right direction to the steering valve regardless of the direction of the rotation of the supplementary pump so that supplementary hydraulic fluid is available for use in steering so long as the vehicle is moving and even when the engine of the vehicle is not in operation." One major difference between the Joyce patent and the present disclosure is that the supplementary pump of Joyce is always directing fluid to the steering valve whenever the vehicle is moving. In the present disclosure, the auxiliary pump does not deliver fluid to the steering valve unless the primary pump is not providing fluid at a predetermined flow to the valve.
It is an object of the present invention to provide a ground drive safety steer pump circuit which has a low horsepower reduction loss.
It is a further object of the present invention to provide a system for providing auxiliary power which responds quickly to a failure of the primary pump.
It is a further object of the present invention to provide a system for providing auxiliary power which is compact.
It is a further object of the present invention to provide a system for providing auxiliary power which is relatively easy to install.
It is a further object of the present invention to provide a system for providing auxiliary power which is efficient.
It is a further object of the present invention to provide a system for providing auxiliary power which is inexpensive to manufacture.